1. Field of the Invention
The present invention relates to a reference track detection for a disc storage unit in which a reference track such as the No. 0 track, i.e., the radially outermost track specified from a plurality of tracks defined on a surface of a disc in a disc storage unit such as a fixed disc storage unit is detected, and more particularly to a reference track detecting method and a disc storage unit employing the method in which servo information for detecting a position of a head is written on one surface of a disc and which has a polyphase head driving motor mechanically and closely coupled to the head to displace the head, vector designating means for designating a vector to be defined by phase currents flowing through the head driving motor thereto and servo information reading means for reading out contents of the servo information through the head.
2. Description of the Prior Art
In the disc storage unit, writing or reading of information is performed by positioning on a desired track. This desired track is designated by a track number generally called a cylinder number. When the head is positioned on a desired track in the disc storage unit, the difference between the cylinder number of the desired track and the cylinder number of the track on which the head is positioned at present is computed. Thereafter, the head is displaced radially inwardly or outwardly by the number of tracks corresponding to the difference in cylinder numbers thus computed. To this end, the disc storage unit always stores the cylinder number of the track on which the head is now positioned, and upon completion of the displacement of the head, the stored cylinder number is replaced by a new cylinder number so that the head is ready to be displaced again. When the reference value of the cylinder number is erroneous or is erased, correct reading or writing cannot be carried out. Accordingly, in the disc storage unit, it is required that the stored cylinder number be calibrated periodically to the cylinder number of the track on which the head is positioned, for instance, whenever the disc storage unit is energized.
In order to carry out the calibration described above, a specific track such as the above-described No. 0 track of a plurality of tracks defined on the surface of the disc is designated as a reference track. In the case of the calibration, after the head is properly positioned on the reference track, the stored cylinder number is replaced by the cylinder number assigned to the reference track. However, in order to properly position the head on the reference track in the case of the calibration, it is necessary to provide suitable means for confirming or detecting that the head is properly positioned on the reference track. To this end, a photointerruptor has long been used. As is well known to those skilled in the art, the photointerruptor utilizes the phenomenon that when an object crosses a very fine beam, the light is interrupted. When the photointerruptor transmits a signal representative of the interruption of the light while the head is being displaced, for instance, in the radially outward direction, the displacement of the head is immediately interrupted and the position at which the head is stopped is defined as a reference track position.
In general, the photointerruptor is incorporated into the disc storage unit in such a way that a light beam is interrupted by the head. In practice, the head is mounted on a carriage, and hence in general the photointerruptor is positioned in such a way that the light beam is interrupted by the carriage itself or by a small part attached thereto. It is, of course, apparent that in addition to the photointerruptors of the type described above, any position sensor with a high degree of detection accuracy may be used to detect the reference track.
When the photointerruptor is used, it is possible to detect a position with a high degree of accuracy by reducing the cross section of the light beam to a minimum, but the photointerruptors are not necessarily inexpensive. Especially, in the recently developed disc storage units, there is the tendency of decreasing the track pitch to make it as small as possible in order to increase the data storage density. Under the circumstances, it is required to increase the degree of detection accuracy accordingly. As a result, it becomes difficult to reduce the cost of the photointerruptor. Apart from the high cost of the photointerruptor, fine adjustment of the position of the photointerruptor with an extremely high degree of accuracy of the order of 10 .mu.m is required when the photointerruptor is incorporated into the disc storage unit. This fine ajustment causes additional cost and a longer work time. Even if any type of position sensor is used instead of the photointerruptor, the above-described problems essentially remain unsolved.